1. Field of the Invention
This invention relates to a III–V group nitride system semiconductor substrate.
2. Description of the Related Art
Nitride system semiconductor materials such as gallium nitride (GaN), indium gallium nitride (InGaN) and gallium aluminum nitride (GaAlN) have a sufficiently wide bandgap and are of direct transition type in inter-band transition. Therefore, they are a great deal researched to be applied to short-wavelength light emitting device. Further, they have a high saturation drift velocity of electron and can use two-dimensional carrier gases in hetero junction. Therefore, they are also expected to be applied to electronic device.
With silicon (Si) or gallium arsenide (GaAs) which is already in popular use, an epitaxial growth layer of silicon (Si) or gallium arsenide (GaAs) to compose a device is homo-epitaxially grown on Si substrate or GaAs substrate of same kind of material. In homo epitaxial growth on homo-substrate, the crystal growth proceeds in step flow mode from the initial stage. Therefore, it is easy to obtain flat epitaxially grown surface while generating little crystal defect. In the case that a ternary or more compound crystal layer such as AlGaInP is grown on GaAs substrate with a lattice constant close to that layer, the surface morphology of epitaxial layer is likely to be roughened. However, by tilting the planar orientation of underlying substrate from low index surface as reference intentionally to a specific direction, which is generally called “off-orientation”, it becomes possible to obtain flat epitaxially grown surface while generating little crystal defect.
On the other hand, it is difficult to grow bulk crystal of nitride system semiconductor and, recently, GaN self-standing substrate with a level for practical use is just developed. At present, a widely used substrate for epitaxial growth GaN is sapphire. The process of growing a nitride system semiconductor epitaxial layer to compose a device is generally conducted as follows. At first, GaN is hetero-epitaxially grown on single-crystal sapphire by using vapor-phase growth such as MOVPE (metal organic vapor phase epitaxy), MBE (molecular beam epitaxy) and HVPE (hydride vapor phase epitaxy). Then, the nitride system semiconductor epitaxial layer is grown on GaN sequentially or in another growth vessel.
Since the sapphire substrate has a lattice constant different from that of GaN, single-crystal film of GaN cannot be obtained by growing GaN directly at a high temperature on the sapphire substrate. Thus, a method is invented that AlN or GaN buffer layer is in advance grown on the sapphire substrate at a low temperature of 500° C. or so, thereby reducing the lattice strain, and then GaN is grown on the buffer layer (e.g., Japanese patent application laid-open No. 4-297023). With such a low temperature growth buffer layer, it becomes possible to obtain single-crystal epitaxially grown GaN. However, even in this method, the lattice mismatch between substrate and grown crystal is not eliminated and, at the initial step of growth, the crystal growth proceeds in three-dimensional island growth mode (Volmer-Waber growth mode), not in step flow mode (Stranski-krastanov growth mode) aforementioned. Therefore, GaN thus obtained has a dislocation density as many as 109 to 1010 cm−2. Such a defect causes a problem in fabricating GaN system device, especially LD or ultraviolet emission LED.
In recent years, ELO (e.g., Appl. Phys. Lett. 71 (18) 2638 (1997)), FIELO (e.g., Jpan. J. Appl. Phys. 38, L184 (1999)) and pendeoepitaxy (e.g., MRS Internet J. Nitride Semicond. Res. 4S1, G3.38 (1999)) are reported that are methods for reducing the defect density generated due to lattice mismatch between sapphire and GaN. In these methods, a SiO2 patterning mask is formed on GaN grown on sapphire substrate, and then GaN is selectively grown from the window of mask. Thereby, the propagation of dislocation from underlying crystal can be suppressed. Due to such a growth method, the dislocation density in GaN can be significantly reduced to a level of 107 cm−2 or so. For example, Japanese patent application lain-open No. 10-312971 discloses such a method.
Further, various methods of making a self-standing GaN substrate are suggested that a thick GaN layer with reduced dislocation density is epitaxially grown on hetero-substrate such as sapphire and then the grown GaN layer is peeled from the underlying substrate (e.g., Japanese patent application laid-open No. 2000-22212). For example, Japanese patent application laid-open No. 11-251253 discloses a method of making a self-standing GaN substrate that a GaN layer is grown on sapphire substrate by ELO and then the sapphire substrate is removed by etching. Other than this, VAS (Void-Assisted Separation: e.g., Y. Oshida et al., Jpn. J. Appl. Phys. Vol. 42 (2003) pp. L1–L3, Japanese patent application laid-open No. 2003-178984) and DEEP (Dislocation Elimination by the Epi-growth with inverted-Pyramidal pits: e.g., K. Motoki et al., Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L140–L143, Japanese patent application laid-open No. 2003-165799) are known. VAS is conducted such that GaN is grown through TiN thin film with mesh structure on substrate such as sapphire while providing voids at the interface of underlying substrate and GaN layer, thereby enabling simultaneously the pealing and the reduction of dislocation of GaN substrate. DEEP is conducted such that GaN is grown on GaAs substrate, which is removable by etching, by using SiN patterning mask while intentionally forming pits surrounded by facets on the surface of crystal, accumulating dislocations at the bottom of pits to allow regions other than pits to have low dislocation density.
However, the conventional methods of making GaN substrate have next problems.
As described, GaN epi-layer to compose a device is once at least hetero-epitaxially grown on hetero-substrate such as sapphire and GaAs with a considerably different lattice constant. This is common both in using GaN template and in using self-standing substrate.
When GaN is grown on a substrate with considerably different lattice constant, a number of small GaN nuclei are generated and then, according as they are grown, neighboring crystal nuclei are coalesced and finally provides continuous film. Thus, the crystal growth proceeds in so-called three-dimensional island growth mode. There is no problem when the growth orientation of individual crystal nucleus is just aligned and the distance between nuclei is just matched with an integral multiple of lattice constant of GaN. However, in general, there occurs a phenomenon that the crystal nucleus is tilted or twisted to the surface of underlying substrate, and the planarization forcedly proceeds while generating crystal defect such as dislocation at the interface of coalesced nuclei. Therefore, GaN grown layer is, though it appears as homogeneous crystal, exactly composed of a number of columnar crystal grains (sub-grains), and a number of defects are contained at the boundary of grains. This is the reason why, in case of GaN, only substrate with a dislocation density as many as 105 to 109 cm−2 can be obtained. In case of Si or GaAs, single-crystal substrate with no dislocation or with a dislocation density of less than 1×103 cm−2 can be easily obtained. Thus, in case of GaN, although the crystal orientation of individual grain is roughly aligned, it has a considerable variation. Although the number of nuclei generated at the initial stage of crystal growth can be significantly reduced by using the aforementioned ELO, there occurs a strain in grown GaN due to a difference between region with mask for selective growth and region without the mask and, thereby, the crystal axis is tilted. As a result, GaN crystal thus obtained still has a variation in grain crystal orientation.
This situation will be explained below with reference to drawings.
FIG. 1 is an illustrative cross sectional view showing the inclination of crystal axis in an ideal substrate 5 with no variation in crystal orientation. Arrows 6 in FIG. 1 are vectors indicating the direction and amount in inclination of crystal axis. A cell including one arrow corresponds to a grain 7. In case of the substrate 5 with c-face on its surface, arrow 6 represents the direction and amount in inclination of c-axis. That, as shown in FIG. 1, there is no inclination of crystal axis in plane of substrate means that no boundary between grains 7, 7 exists in fact. It is presumed that Si substrate has such a structure.
FIG. 2 is an illustrative cross sectional view showing the inclination of crystal axis in an ideal substrate 8 with “off-orientation”. Arrows 9 indicate the direction and amount in inclination of crystal axis of individual grain 10.
FIG. 3 is an illustrative top view showing the in-plane distribution of inclination of crystal axis in the ideal substrate 8 with “off” orientated while viewing the arrows 9 from the surface side of substrate 8. Arrows 11 indicate the inclination direction of crystal axis. If there is an ingot with crystal axis aligned, the substrate with a distribution of crystal axis as shown in FIGS. 2 and 3 can be easily made by cutting diagonally and polishing it. It is presumed that “off” substrate generally used for GaAs or sapphire substrate has such a distribution of crystal axis.
FIG. 4 is an illustrative cross sectional view showing the inclination of crystal axis in a conventional GaN substrate 12. Arrows 13 are vectors indicating the direction and amount in inclination of crystal axis.
FIG. 5 is an illustrative top view showing the in-plane distribution of inclination of crystal axis in the substrate 12. Arrows 15 indicate the inclination direction of crystal axis. As described earlier, in crystal growth of nitride system, due to the three-dimensional island growth where hetero-epitaxial growth is forced, there are a number of grains 14 in the crystal substrate. These grains 14 are aligned along nearly equal crystal axes and the entire substrate is formed as close as single crystal. However, the crystal axis of each grain 14 is inclined by tilting or twisting and the crystal axes in plane of substrate have a variation. Especially in the substrate 12 with no “off-orientation”, the inclination direction of axes of grains 14 has to be varied in disorder.
Thus, in the case of having a variation in the direction and amount of inclination in crystal axes in plane of substrate, a flat epi-surface cannot be obtained even in homo-epitaxial growth when GaN is epitaxially grown on such a substrate.
In general, crystal growth rate has anisotropy to crystal orientation. Therefore, if the inclination direction of crystal axes is different or the amount of inclination is significantly varied on the surface of substrate, there occurs a difference in crystal growth rate by location even in plane of substrate. Thus, film with uniform thickness cannot be obtained. Further, step bunching is generated locally and it causes a roughness in morphology of epi-surface. This is a problem specific to nitride system semiconductor material and not common to conventional semiconductor materials such as Si and GaAS.
The roughness in morphology of epi-surface is amplified according as the thickness of epi-layer increases. Therefore, although the roughness in morphology is slight at the stage of having only one thin GaN layer on sapphire substrate, the roughness of surface is amplified when an epi-layer is further grown using that layer as template. This causes a reduction in product yield of device fabrication process and in characteristics of device itself.
In case of GaN self-standing substrate, based on the same reason, the surface of GaN generally has unevenness such as big undulation or hillock when thick GaN layer is grown on hetero-substrate. When using it as GaN substrate, its surface is mirror-finished by polishing. Thus, it appears to have a sufficiently flat surface. However, since the crystal itself is composed of grains with tilting or twisting as mentioned earlier, regardless of polishing, the roughness in surface morphology will occur due to the underlying grains when GaN layer is further epitaxially grown on the polished surface of GaN substrate.
In the conventional methods, there is suggested a thought that the variation of planar orientation in GaN single crystal substrate is defined. For example, Japanese patent application laid-open No. 2000-22212 discloses a GaN substrate that the inclination and variation of planar orientation is defined. However, it aims at eliminating influences by the warping of substrate. In other words, the conventional methods do not have an idea that the inclination direction is to be aligned in plane of substrate. Thus, they offer no solution to the problem, i.e., roughness in surface morphology. For example, this is proved with reference to FIG. 15 of No. 2000-22212.